Understanding Industrial Blueprints 187
Handedness.
Almost all threaded fasteners tighten when the head or nut is rotated clockwise. That is, as a viewer turns
a nut clockwise it moves away from her. Such a fastener is said to have a right-hand thread; all screw
fasteners are assumed to be right-hand unless otherwise specified. Left-hand threads are usually found
only on rotating machinery. For example, the axles of bicycle pedals screw into threaded holes in the
cranks. In a pair of pedals one will have a right-hand thread and the other a left-hand thread. That way
the rotation of the pedals doesn’t tend to unscrew their axles. To designate a left-hand thread, the let-
ters “LH” are placed after the class of fit: 3/8-16 UNC 2B LH (21).
Helix.
A helix denotes the curve formed on any cylinder by a straight line in a plane that is wrapped around the
cylinder with a forward progression. Common objects formed like a helix are a spring, a screw, and a
spiral staircase.
Helices can be either right-handed or left-handed. With the line of sight being the helical axis, if
clockwise movement of the helix corresponds to axial movement away from the observer, then it is a
right-handed helix. If counterclockwise movement corresponds to axial movement away from the ob-
server, it is a left-handed helix.
External Thread.
An external thread, also known as a male thread, is a thread on the outside of a cylinder or cone. An ex-
ample is the thread of a bolt.
Internal Thread.
Also known as a female thread, an internal thread is a thread on the inside of a hollow cylinder or bore.
An example is the thread inside a nut.
Major and Minor Diameters.
A major diameter is the largest diameter of an external or internal screw thread. A minor diameter is the
smallest diameter of an external or internal screw thread; it is also known as the “root diameter.”
Cut Thread.
Screw threads are cut or chased (to cut thread in a lathe rather than with a die); the unthreaded portion
of the shank will be equal to the major diameter of the shank.
Axis.
The axis is the center line running lengthwise through a screw—the line, real or imaginary, passing
through the center of an object about which it could rotate, or a point of reference.
Crest.
The crest is the top or outer surface of the thread joining the two sides (also called “flat”). It is the sur-
face of the thread corresponding to the major diameter of an external thread and the minor diameter of
an internal thread.